Method and device for the continuous analysis of the composition of a gas



Aprll 7, 1970 J. LUCKERS 3,504,521

METHOD AND DEVICE FOR THE CONTINUOUS ANALYSIS OF THE COMPOSITION OF AGAS Filed Oct. 25, 19s? VOLTAGE B o zo' lo FlG.l. E

INVENTOR JOHN LUCKERS BY \um ATTORNEYS United States Patent METHOD ANDDEVICE FOR THE CONTINUOUS ANALYSIS OF THE COMPOSITION OF A GAS JohnLuckers, Liege, Belgium, assignor to Centre National de RecherchesMetallurgiques, Brussels, Belgium, a

corporation of Belgium Filed Oct. 23, 1967, Ser. No. 677,481 Claimspriority, application Belgium, Oct. 25, 1966, 41,119, Patent 688,861Int. Cl. G01n 31/00 US. Cl. 73--1 6 Claims ABSTRACT OF THE DISCLOSURE Agas analyser generates signals and passes them to a calibration unit andthence to a recorder. The calibration unit establishes at least twopoints on a variable characteristic by analysing separate standard gassamples. The analysis of one standard gas will correct the zero of thecharacteristic of the calibration unit and the gain characteristic ofthe calibration unit is corrected while analysing the other standard gassample. The values of the corresponding points to the established pointsof the standard gases are then brought into correspondence and analysiof the standard gases is repeated at intervals.

The present invention relates to a method and a device for thecontinuous analysis of the composition of a gas, in particular of ablast furnace gas.

It is well known to analyse gaseous products of chemical orphysico-chemical reactions for the purpose of obtaining information onthe exact nature, the rate and the state of progress of the saidreactions.

Equipment is available for analysing gases, in particular the throatgases of a furnace, which function satisfactorily. For this functioningto be considered as satisfactory, it is however necessary for a certainnumber of conditions to be fulfilled, among which can be mentionedprimarily:

(1) Strict selection of measuring instruments, as well as of componentsassociated with these instruments;

(2) Careful choice of the surroundings (temperature, vibrations,humidity) in which the said instruments are set up;

(3) The necessity for ensuring that the supply of sample gas to theanalyser is of constant humidity and pressure that is stabilised at afixed absolute pressure.

However, it has been found that even when all these conditions arefulfilled it is necessary to cary out periodical calibration of theoutlet signals of the analysers, by using standard samples of gases. Forthis reason gas analysis equipment comprises not only an analyser, butalso a calibration unit, which adjusts the analyser periodically, and aregistration member on which the indications supplied by the analyserare recorded.

In Luxembourg Patent 50,379, there has already been proposed acontinuous method for the analysis of gas in which the analysers areadjusted periodically, the adjustment or calibration being carried outpreferably automatically.

This automation can be realised effectively by means of an electronicordinator, but if such equipment is not available, an independentcalibration unit can be used.

Such calibration units are well known but they often have disadvantagesarising either from the principle used, or the quality of production.Among these disadvantages the following can be mentioned by way ofexample:

(1) Cross influencing between the zero and gain corrections, i.e.between the regulating of the slope of the characteristic (gain) and theregulating of its co-ordinates at origin (zero).

(2) Insufiicient precision in the apparatus used;

(3) Very high cost of this equipment in relation to the performanceobtained.

The aim of the present invention is accordingly to overcome thesedisadvantages and to supply a method for adjustment which allowsindependent zero and gain corrections, as well as increased precision.

The method for the automatic adjustment of gas analysers which forms thesubject of the invention has the essential feature that thecharacteristic curve of the calibration unit is modified as a functionof the drift of the analysis apparatus, in such a way as to maintaininvariable indications on the recording apparatus, and that thismodification is obtained by means of zero and gain corrections which areabsolutely independent of each other.

The invention also aims at providing a device, called hereinaftercalibration unit, by which it is possible to put in operation the methodof the invention.

For the better understanding of the method of the invention, referencewill now be had to its application to the automatic adjustment orcalibration of a blast furnace gas analyser which is described withreference to the accompanying drawings. This description is given solelyby way of example, and in no restrictive sense.

In the drawings:

FIGURE 1 shows the principle of the modification of the characteristicof the calibration unit during the correction of the drift of theanalysers;

FIGURE 2 shows the functioning diagram of an automatic calibration unit.

The calibration of a gas analyser, for instance an infrared gasanalyser, includes the control of at least two points on its scale.These two points are preferably located at or near the limits of thenormal range of variation of the gaseous analysed component.

Graph 0, FIGURE 1A, gives the initial characteristic of the analyser,which is taken as a reference characteristic, obtained from indicationsU and U supplied by the analyser for the standard sample gases G and GThese signals U and U applied to the input of the calibration unit(FIGURE 1B) give output signals 0 and 0 which makes it possible to plotthe graph a (FIGURE 1B). In the same way, these signals 0 and 0introduced into the recorder, by virtue of the characteristic of thisapparatus give indications G and G which should repeat when the analyserfunctions with one of the sample gases G or G respectively (FIGURE 1C).

It should be noted that these calibration graphs can be considered asstraight lines, by reason on the one hand of the limitation of themeasurement range used and on the other hand of the impossibility ofactually checking the real curvature with sufficient precision, sincethe deviations of the real graph line in relation to a straight line areat a maximum of 0.1 volume percent.

After a certain period of operation, the analyser A will have drift andthe gases G and G give respectively the indications U and U (FIGURE 1A,graph b). Under these conditions the characteristic a of the calibrationunit gives output signals 6 and 0 which no longer supply the correctindications G and G to the recording member.

In order to re-establish these indications G and G it is accordinglynecessary to modify the characteristic of the calibration unit. Thismodification is possible, in accordance with the invention, by means ofa single control for each of the two gases analysed G and G theprocedure being as follows:

(a) During the passage of the gas G in the analyser A, one subtracts avoltage A from U at the input of the calibration unit, in such a way asto cancel out the output signal 6 In FIGURE 1B, this corresponds to thetranslation of the characteristic of a into a.

(b) The analyser is changed over and the gas G caused to pass; duringthe passage of the latter, there is picked up at the output of anamplifier A a voltage which is defined as (0 -9 i.e. the differencebetween the input signals to the calibration unit arising from thestandard gas G and G These two voltages 6' and 0' should be renderedequal. To arrive at this, their diflference (6' 9' is reduced to zero,which, as can be seen from FIGURE 1B, reveals itself by the changerotation of the characteristic graph from c to d.

(c) As the output voltage of the calibration unit should be 0 and notzero for the gas G it suffices to add a voltage 6 to the output signalsof the calibration unit.

For the gases G and G the output voltages accordingly becomerespectively 0 and whereas the input voltages are U and U The newcharacteristic is accordingly determined and in FIGURE 1B, the graph dexecutes a translation of 0 to arrive at b, which is the graph desired.A further passage of the gas G in the analyser will give at the outputof the calibration unit a voltage 0 which clearly shows that theregulation of the gain of the calibratiion unit has not alfected theregulation of zero carried out previously.

All the operations mentioned above are carried out by the automaticcalibration unit, of which, FIGURE 2 shows a circuit diagram.

This assembly has two operational amplifiers AI and A2 which carry outthe various operations additions of voltages, of modifications of gainand of comparison to reference voltages.

Three miniature synchronous motors (M1, M2, M3) control respectivelypotentiometers P1, P2, P3 for regulating the gain for regulating zeroand for normal indication of operation in a blast furnace, respectively,a potentiometer P4 coupled to the shaft of the motor M3 at the same timeas P3 gives the signal supplied to the recording member. The control ofthe motors is under the control of a device R sensitive to the amplifierA2 and consists in a reversal of their direction of operation. This ispossible by virtue of the fact that each method of functioning consistsin bringing the output voltage of A2 to zero, i.e. to an equilibriumwhich can take place solely on an indication of imbalance.

The resistors R3 through R9 modify the signals applied in dependence onthe operating characteristics of the amplifiers A1 and A2.

Manually controlled potentiometers P5 and P6 supply variable signals.They are set to positions corresponding to output voltages equal to 6and (0 0 respectively.

The gain K of the amplifier A1 is controlled by the resistors R1, R2 andthe potentiometer P1, which is controlled by the motor M1.

The circuit further comprises eleven relays numbered 1 to 11, each ofwhich has one earthed or open position and one closed or operativeposition.

Relays 1 to 6 are actuated by the control unit for the calibrations.Relays 7 to 11 are actuated by manual control. They provide thefollowing functions:

(a) Functioning on blast furnace gas The relays 1, 2, 4, 8, 9, are open.

The relays 3, 5, 6, 7, 10, 11 are closed.

The amplifier A2 accordingly receives the voltages 0'=K(UA), 0 and C.R.,where K is a constant of proportionality representing the gain of A1.The output voltage of A2 is cancelled permanently by the action of themotor M3 on the counter voltage GR: in other words, the voltage GR. iskept constantly equal to On the other hand, the voltage E supplied tothe recording member R is supplied advantageously from a separatepotentiometer P4, coupled to the shaft of the motor M3 in such a way asto avoid any influence of the recording member on the gas analyser. Asthe angular position of the shaft of this motor varies as a directfunction of 6', the voltage E is also proportional to 8, whichcorresponds to the characteristic indicated in FIG- URE 10.

(b) Functioning on sample gas 6,: correction of zero to compensate fordrift The relays 1,3, 4, 5, 6, 8, 9, are open.

The relays 2, 7, 10, 11, are closed The amplifier A1 receives U and A,and the motor M2 acts on P and hence A in such a way as to make theoutput voltage of A1, equal to 6 This corresponds, in FIGURE IE, to thetranslation of the characteristic from a at c.

(c) Functioning on sample gas G correction of gain to compensate fordrift The relays 2, 3, 5, 6, 8, 9, are open.

The relays 1, 4, 7, 10, 11, are closed.

The amplifier A2 receives K(U A) and (6 6 its output voltage is renderedzero by varying the gain K of A1, i.e. by making K(U 'A)=0 0 The gain Kis dependent on the setting of the potentiometer formed by resistor R2and potentiometer P1 and this is variable by means of the motor M1.

This annulment of the output voltage of A2 corresponds to the rotationof the characteristic from c at d (FIGURE 1B).

(d) Further functioning on blast furnace gas The relays 1, 2, 4, 8, 9,are open The relays 3, 5, 6, 7, 1t), 11, are closed.

With the relay 5 again closed, one adds a voltage 0 to the output signalof A1, which (FIGURE 1B) means the translational displacement of thecharacteristic from a" to b.

The use of a recording member supplied by an independent voltage sourcepresents a considerable supplementary advantage. In actual fact, duringthe adjustment of the calibration unit by means of standard samplegases, the recording member during the whole period of the adjustmentmaintains the indications obtained during the functioning of theanalyser with blast furnace gas, which makes it possible for theoperator to exploit these indications to the maximum, in particular forintegration by the planimetering of plotted graphs. These indicationsare modified only during the subsequent functioning of the analyser withblast furnace gas.

(e) Regulation of references For undertaking the regulation of thereference voltages 6 and 0 i.e. of signals corresponding to the low andtop of the scale gases, it has been found advantageous to work in thefollowing way:

Regulation of 0 one opens the relays 1, 2, 4, 7 and one closes therelays 3, 5 and 6. By reason of the opening of the relay 7, thepositions of the relays 8 to 11 have no importance, since the connectionfrom A1 to A2 is interrupted. By putting M3 in operation with 6 at theinput of A2, the recorder indicates a value proportional to 6 As thisvoltage 9 is supplied from a potentiometer (P5) with manual control, itis easy to bring about the coincidence of the indication on the recorderwith the already known line corresponding to the analysis of thestandard or sample gas G This already known line may be marked on therecorder scale for ease of reference.

Regulation of 0 one opens the relays 1, 2, 4, 10, 11 and one closes therelays 3, 5, 6, 7, 8 and 9.

The values of the resistors R and R are such that the gain of theamplifier A1 is 1, for this method of functioning. Accordingly, at theoutput of the amplifier A1, one has a voltage (6 6 with changed sign,i.e. (0 -0 The input voltage of the amplifier A2 is and the motor M3begins to operate, carrying the potentiometer P4. The recorderaccordingly indicates a value proportional to 0 By modifying manuallythe position of the potentiometer P6, it is easy to bring about thecoincidence known position of the indication on the recorder with theline corresponding to the analysis of the standard gas G I claim:

1. In a method for the adjustment of gas analysis apparatus comprising agas analyser, a calibration unit and a recorder, in which the gasanalyser generates signals and passes them to the calibration unit, thecalibration unit having a variable characteristic: the improvement ofestablishing at least two points on the characteristic by analysingseparate standard gas samples, repeating the analysis at intervals andindependently correcting the zero of the characteristic of thecalibration unit while analysing one of the standard gas samples and thegain characteristic of the calibration unit while analysing the otherstandard gas sample so as to bring the values corresponding to thestandard gases into correspondence with the established points.

2. A method as claimed in claim 1, in which the recorder is supplied bymeans of a separate voltage source so as to avoid interactions betweenrecorder and analyser.

3. In apparatus for gas analysis comprising a gas analyser, acalibration unit adapted to receive signals from the analyser, and topass modified signals to a recorder,

the calibration unit comprising means for modifying the characteristicthereof: the improvement that the modifying means includes means formodifying the gain of the calibration unit and means for modifying thezero of the calibration unit to compensate for drift in the gasanalyser, means responsive to signals received from the analysis of afirst standard gas to set the said means for modifying the zero of thecalibration unit and means responsive to signals received from theanalysis of a second standard gas to set the gain of the calibrationunit, whereby the output signals of the calibration unit are restored tostandard values for the tested gas.

4. Apparatus as claimed in claim 3, comprising means for disconnectingthe means for setting the zero and gain characteristics of thecalibration unit whereby the calibration unit may be used with the gainand zero correcting means stabilized at the selected settings.

5. Apparatus as claimed in claim 3, comprising an independent source ofpower to the recording unit.

6. Apparatus as claimed in claim 3, in which modifying means for gainand zero function automatically.

References Cited UNITED STATES PATENTS 2,400,828 5/1946 Keinath 346-322,790,945 4/1957 Chope 3309 XR 2,919,409 12/1959 Williams 33010 XR3,292,421 12/1966 Meyer 73-27 3,310,974 3/1967 Banks 731 LOUIS R.PRINCE, Primary Examiner H. C. POST III, Assistant Examiner US. Cl. X.R.73-23; 3309

